Why max Q on Taric?

One thing really bothers me, when i see other people, especially pros, play Taric:

I always see them max out Q over W or even E, whoch really makes no sense to me...

I main Taric, and I play good with him, my winrate is good and I normaly get at least an A rating after games, unless the game was really terrible for my team and we got destroyed super hard, and until now I felt like 2 points in Q are enough to use it to its full potential, since I want to spam the spell to keep my passive up, which means like 80% of the time i will cast it with 2 stacks anyway.

Sure, when my stun comes up, I could have 4 stacks, but that still takes time, and in the meantime, the mana costs would go up alot, and I really want to save my shield until I either have to protect my ADC or put it on the diver who makes a really good engage, so the only spell i really use on CD all the time is Q...

I would really like to know where I am thinking wrong here...